Peter Gray/WEST PALM BEACH
The latest incarnation of Sikorsky's S-76 is the first to meet in one airframe/engine combination the differing performance requirements of the helicopter's two traditional markets: utility and corporate. Previously, the US manufacturer built two variants of the aircraft: a long-range version for the cost-conscious utility market and a high-powered version for the safety-conscious corporate market. With advances in engine technology, however, Sikorsky is able to offer low fuel consumption and good single-engine performance in one variant, the S-76C+.
The Turboméca Arriel 2S1-powered C+ is now the only version of the medium twin-turbine helicopter that Sikorksy produces. The company sees a substantial market for the 12-passenger S-76C+ in the offshore, corporate, emergency medical service, search and rescue, and utility roles. Some 20 aircraft have already been sold, including one for the UK's Royal Flight. Its nearest competitors are the US Bell 412 and Eurocopter's AS365 and new EC155.
Sikorsky invited Flight International to its West Palm Beach, Florida, plant to evaluate the S-76C+. The example flown is a development aircraft which, over its life, has been an A, B and C model prototype and now represents the C+.
COCKPIT FEATURES
This test aircraft does not have the latest "glass" cockpit now available in the S-76, with its electronic flight-instrument system (EFIS) and integrated instrument-display system. This equipment is cheaper and more reliable than conventional mechanical instruments, Sikorsky says. The malfunction display system is computer managed and prioritises events, but it memorises everything - all is recorded and revealed when interrogated. There is still the usual array of "get-home" standby instruments.
The S-76C+ has been certified for day visual flight-rules through to single-pilot instrument flight-rules operation. The latter version comes with a Honeywell SPZ-7600 four-axis (pitch, roll, yaw and collective), dual digital, automatic flight-control system (AFCS) fully coupled with the flight director and autopilot. Other equipment includes dual integrated navigation and communication systems, moving map display, and a weather radar, the display of which can be superimposed on the EFIS. This additional equipment increases empty weight to 3,820kg, but still leaves 1,485kg for payload and fuel.
AFCS capabilities include an automatic instrument approach, with deceleration to 70kt (130km/h) and automatic go-around straight ahead at 75kt and 750ft/min (3.8m/s). By the end of the year, the search and rescue model will be capable of an automatic approach to the hover, and will be equipped with the forward looking infrared sensor and searchlight already approved on the C model.
The test aircraft also does not have a passenger interior. Instead, there is telemetry equipment expected in an experimental aircraft. We were, however, well able to explore the enhanced performance, characteristics, qualities and systems of the S-76C+. After a comprehensive pre-flight briefing from Nicholas Lappos, Sikorsky's assistant chief pilot, and Jeffery Schneider, S-76 programme manager, I was ready to fly.
WEIGHT CHOICE
The day of our flight was hot (33¼C), giving a density altitude of 2,000ft (600m), with 10kt of wind. I was given a choice of take-off weights - maximum gross at 5,300kg (11,675lb) or the training weight of 4,300kg. I chose the latter, to take maximum advantage of the embedded training functions. We took off at 4,500kg, making us fairly heavy for high speed manoeuvring.
Lappos showed me around the aircraft. For a 1980s' airframe design, the S-76 looks sleek and attractive. It is this sleekness, high performance blades, modern engines and ducted engine exhausts that give the helicopter its range and speed. There is the usual radome in the nose, behind which, in an easily accessible compartment, are the avionics boxes. There is another area behind the baggage compartment for any additional equipment.
All the doors were open, revealing the large 5.78m3 (205ft3) cabin, which can accommodate 12 passengers. When the aircraft is flown by a single pilot there is another passenger seat available in the cockpit. The corporate interior (assuming two pilots are carried) has configurations from four to eight seats. The 12/13-passenger utility/offshore version has a lower empty weight of 3,340kg, leaving 1,975kg for passengers and fuel. With full fuel (855kg), this still allows 93kg per passenger (including bags). The emergency medical service model accommodates two patients and four attendants.
The main rotor head is easily accessed via steps and handholds on both sides. We examined the low drag, low maintenance hub, which has a life of about 20,000h. I recognised the anti-vibration bifilars from my days flying Sikorsky's earlier S-61. There are additional vibration absorbers in the nose and cabin. The four main rotor blades are constructed of titanium and composites. Their tips are swept backwards to increase performance and reduce noise.
Lappos explained the redundant tie rod, with its alternate load path and stress relief, which was incorporated into the blade-to-hub attachment after two accidents in the 1980s when a spindle failed and the blade came off. The offset blade hinges allow, among other advantages, a wide centre-of-gravity range, making it difficult to get the balance wrong when loading the aircraft with passengers and freight. The hub, with its elastomeric bearings, requires lubrication only of the dampers. I saw the rotor brake mechanism, which can be used to stop the rotors with both engines at idle.
Safety features appear to have been high on the priorities for the S-76's designers, so the main gearbox has two oil pumps, each capable of delivering enough pressure to continue safe flight. The gearbox also has a 30min dry run capability. The engine cowlings were slid back for my benefit. On production aircraft they are made from lightweight composites - less expensive to make and easier to maintain than the metal cowlings fitted to this prototype. The firewall bulkhead separating the two engines slides back with the cowling, greatly improving access. An engine change can be accomplished in 2-3h, Sikorsky says.
The digitally controlled Turboméca Arriel 2S1 turboshaft has a single-engine emergency contingency rating of 730kW (980shp) for 30s, that vital time needed when an engine fails at an inopportune moment, such as at the rotation point where the pilot has committed the aircraft into the air and cannot land back. I was to explore this very moment when I flew, and I found out that 30s is more than enough time to adjust the rotor RPM, evaluate what has happened, and get the aircraft into a climb, accelerating to the single-engined safety speed, safely clearing all obstructions and then establishing the best rate of climb speed.
A lower power rating of 660kW can be maintained for 2min. Even an inexperienced pilot should be able to sort out the problems and get the aircraft going uphill with a substantial rate of climb on the remaining engine within 2min, providied that he gets his speed right.
In the event of an engine failure, the remaining engine looks after itself, providing maximum power automatically. The warning system informs the pilot at 27s when his 30s at maximum contingency power is up and, similarly, when his 2min limit is approaching. The pilot then has to reduce to single-engined maximum continuous power, by which time he should be nearing the safe obstacle-clearance height. At each power reduction, the pilot has to lower the collective pitch lever slightly to maintain optimum rotor RPM. If he forgets in the stress of the moment, no harm is done; the S-76C+ will fly safely at lower RPM, but the warning system will sound when rotor speed is getting too low.
The engines are controlled automatically, but there is a manual control if all else fails. The main gearbox will take all the power a single engine can produce, which is good news to pilots who fly other helicopters where power has to be restricted because of single-input gearbox limitations. The only gearbox limitation in the S-76C+ is when using both engines at 5min take-off power. This is a common characteristic in multi-engine helicopters, but is not restrictive in the C+ since both engines are capable of providing 1,275kW, although limited to 1,195kW at sea level. This amount of power provides excellent performance, which is retained as the air temperature and/or altitude increases.
NEAT MANUAL
I studied the flight manual and found it neat and compact, unlike that of the S-61, which was untidy and voluminous. The performance graphs allowed me to establish not only the maximum aircraft weight for the prevailing outside air temperature and altitude, but, by using them in reverse, to calculate the maximum public transport weight possible with the take-off and reject distances available.
Trials are continuing on shorter take-off and reject distances for the Royal Flight aircraft, which will operate into areas more confined than the existing 230m field length. Lappos demonstrated a proposed Royal Flight forwards and upwards take-off, with an engine failure at the critical decision point and a single-engined land-back within 170m.
Lappos and I performed some spot checks on actual aircraft performance while airborne and compared them with the flight manual figures. There was no difference. The flight manual shows that Category A performance - that weight, altitude and temperature condition which guarantees continued safe flight or land-back after any engine failure at any time - is available up to 27íC at sea level and up to nearly 600m at ISA conditions.
The S-76C+ will meet Cat A performance criteria up to 5,320kg at sea level. A land-back at maximum weight requires about 460m, or less if the engine power is above minimum specification, which it usually is fresh out of the factory - around +3% is common. Ground level and elevated helideck Cat A techniques have also been developed. The performance graph can be used in reverse to establish the field length required for a particular aircraft weight, ambient temperature and altitude, or the weight can be adjusted to allow use of the existing field length.
An out-of-ground-effect (OGE) hover at take-off power is possible at maximum weight up to 26íC, and at ISA up to 1,750ft. An OGE hover is possible at sea level, in ISA+20íC conditions, at weights up to 5,130kg.
We did some fuel flow versus airspeed checks in the air. These too were identical to the graph figures. Some relevant examples are best range on a standard day at 4,000ft - 127kt at 265kg/h giving a range with full fuel of 760km, and long range cruise at 132kt giving 745km.
The S-76 has two independent hydraulic systems (the aircraft is unmanageable with no servo assistance), including jam-proof separate jacks. The systems management has the same design criteria as the S-61 - it is failsafe and there is no way you can be left with no system unless you have a double hydraulics failure. There are quick disconnects for servicing. The relatively high hydraulic pressure of 207bar (3,000lb/in2) requires good pipe connections and careful maintenance to avoid subsequent leaks.
The baggage compartment is accessible from both sides of the aircraft and holds 270kg within 1m3. The four passenger and pilot doors allow convenient evacuation in an emergency. In the 12-seat configuration, the two back rows of four passengers can each leave through the cabin doors, the front row passengers through the cockpit doors.
We checked the all-composite tailrotor. There are no bearings, so lubrication is not required, and there is no life limit - maintenance is on condition. The tailrotor is extremely effective, with lots of power available, as I was to find out when I asked Lappos to show me how fast the helicopter could be flown sideways. Backwards flight at about 50kt was also impressive, with no tendency for the fuselage to tuck forwards.
Tailrotor pitch control is via cables, but unlike other helicopters with cables, when a break can cause total loss of pitch control, a clever design results in the equivalent of pushing a rope. A single cable failure still allows almost full tailrotor pitch control; a double failure allows less, but it is still manageable. Pitch change is transmitted to the blades through horns which twist the spar.
PLATFORM COVERS
As we continued our walkround, Lappos pointed out the undercarriage covers, which can be used as a two-man maintenance platform. The undercarriage, and indeed the rest of the airframe, is designed to withstand a 20g crash impact with no movement of any primary structure. This means that, in the event of a very heavy landing, the main gearbox and/or engines will not penetrate the cabin, an event which has killed the occupants of other aircraft.
There are no fuel booster pumps for the pilot to manage - the engines suck up the fuel themselves. If the aircraft crashes and fuel lines are broken, there will be no leakage of fuel, just the reverse - the fuel system will take in air. Each engine has its own fuel supply system, but there is a crossfeed which allows either powerplant to be fed from either tank or both engines to be fed from one tank.
I noted the double latching system on all the external panels. This prevents a panel opening in flight and flying off. This can be catastrophic in helicopters if the offending panel then hits a rotor. Offshore operators are provided with push-out windows, essential for getting out quickly from a ditched and inverted helicopter. The flotation system has total redundancy and the inflation bottles are in the cockpit floor.
I noted that the external power socket, although on the pilot's side, is out of view, so care must be taken to ensure it has been disconnected before take-off. There is a warning light, but nothing is more effective than looking to make sure it is disconnected.
I climbed easily into the right-hand pilot's seat and fastened the five-point harness. On this test aircraft, both the pilots' seats are crash attenuating and such a design is being contemplated for production aircraft. The seat is adjustable vertically and pedals can be moved through a considerable longitudinal range. I was able to find a combination giving me good reach and the ability to see over the nose, as Lappos recommended. This did not allow me to keep the warning lights at the top of the instrument panel in view, so I had to duck my head slightly. The cockpit is roomy and comfortable even though this is a test aircraft.
I started the first engine by moving the overhead speed select lever forward into idle, pressing the starter button on the lever and releasing it. The start, like all other engine functions, is fully automatic except for deliberate selection of manual throttle for training or a malfunction. We observed the cool, slow engine start.
Lappos got everything on line quickly and I taxied out using just a small amount of forward cyclic stick and no collective lever, so producing very little downwash. This was proven when we had to go quite close to a small fixed-wing aircraft - not a control surface moved. My first hover was uneventful and required few control inputs from me to keep us stationary over the spot. The torque and other power presentations were excellent.
All the usual hover manoeuvres were equally uneventful until I asked Lappos how fast we could go sideways and backwards. He took control and achieved 50kt sideways to the left with still some right pedal available; 55kt to the right used nearly all the left pedal. We went backwards at 50kt. There was no tendency for the nose to tuck under. This is good news for offshore pilots who have to perform out-of-wind take-offs and landings in strong winds. This aircraft has no control problems in the hover with winds coming from all directions.
We went to 6,000ft for some general handling and steep turns. No pedal is required to balance turns above 60kt. We pulled 1.7g in a 60í banked turn. There was no discernible increase in vibration. Visibility in the turn was good. I handed over control so that Lappos could demonstrate vigorous pulls and pushes of the collective lever to show the engines' load sharing characteristics and governing response. He was brutal, but power turbine and rotor RPM varied by no more than 1%.
UPHILL POWER
While still fairly heavy, we went to the Vne (never-exceed speed) of 135kt. The sea level Vne is 155kt and starts to reduce at 3,000ft density altitude. We achieved Vne going uphill. This is remarkable and demonstrates the power available from the new engines as well as the low drag of the fuselage.
I liked the spring trim device which holds the lever where you put it, with a release trigger underneath the collective head. The stabilisation equipment holds the aircraft in whatever attitude chosen, with a "coolie hat" switch on the cyclic stick head to make small adjustments.
I flew the aircraft raw with no stabilisation. In the climb there was self-induced roll and yaw; in a descent these disappeared. High speed at a lower level brought back the roll and yaw and I had to work harder to fly the aircraft accurately, but I am sure that after a little practice, one would be able to fly quite accurately. As soon as I went into a turn, the rolling and yawing ceased.
We returned to base and I did a very steep approach, probably about 60í. There was a slight, but brief, airframe vibration as we decelerated through translational lift. This is a typical Sikorsky phenomenon. The visibility was excellent, although I had to pedal the nose just slightly to the left out of my downwards and forward vision to see my landing area.
The S-76C+ has a sophisticated training mode built in to the full authority digital engine control. Lappos selected it and immediately the aircraft simulated the maximum Category A weight for our ambient temperature and height. I saw the compressor speed and torques rise to what they would be if we were at maximum Cat A weight, giving us 10% torque in hand. The T5 temperaturse and fuel flows stay true. The take-off technique is uncomplicated - a 1.5m wheel height hover, add 10% torque, accelerate level to the pre-calculated critical decision point (CDP), at which point the attitude is changed to climb and accelerate to V2 (CDP +10kt), then to the best rate of climb speed (Vbroc).
At the CDP, Lappos selected one engine inoperative. I had an immediate rotor droop from 107% to below 100% so had to lower the lever to restore the RPM to at least 100%. The remaining "good" engine went automatically to simulated 30sec power rating, the warning light came on and the internal clock started to run. In reality, because we were at the much lower training weight, the "good" engine went to only 100% N1 (maximum continuous power), although the instrument showed over 104% as it would in reality. Similarly, the torque indication went to over 130%, although in reality it would not go above maximum continuous.
We accelerated to V2 (the best single engine speed) and achieved 350ft/min. At 27sec the flashing warning light indicated that we were approaching the engine time limit. I pressed the limiter switch on the end of the collective lever and the engine reverted to 2min power. I had to lower the lever slightly to retain 100% rpm. The internal clock restarted. The acceleration from V2 to Vbroc cost us our rate of climb, which went to zero but then back up to 400ft/min as we achieved Vbroc. This transition requires delicate handling, training and confidence, especially in instrument conditions.
At 1min 57sec, the warning light flashed again and I pressed the limiter to reduce the power to maximum continuous. Another small lever reduction was required to retain the RPM.
The excellent training mode allows all the required practice without using real emergency power, but gives the effects of a fully laden aircraft. Several built-in safety features automatically bring in full power in the event of a malfunction or poor pilot handing. Next Lappos again failed an engine at CDP and I flared and landed back well within the published reject distance. I got him to call out rotor RPM and torque, but neither approached any limits.
The whole system is well designed to reduce the pilot workload during such critical manoeuvres. All he needs to do is observe the time limit warning system, use the lever to control rotor RPM, the limiter switch to control the power and get his airspeed correct. There is no need to commit to memory a lot of engine speed, temperature and torque figures. The single engine mode can be selected at any time during all phases of flight. The pilot has to remain vigilant to restore any lost RPM during reversion and get his airspeed correct - just like the real thing.
MANUAL THROTTLE
The other engine training mode which I investigated was manual throttle, particularly with overhead speed select levers and single pilot mode. Lappos selected the simulated major malfunction of the automatic engine management system, which brought on a blue warning light in the appropriate speed select lever and another warning light at the top of the console in my field of vision. The engines are designed to "freeze" in the event of an automatic control failure and not run up or down.
After stabilising in straight and level flight, I raised and lowered the lever slightly to confirm that the number two engine had frozen, then reached up and took the appropriate speed select out of its "fly" gate and used the usual technique of reducing its torque to below that of the "good" engine. Cruise required no further adjustment.
Now for the approach and landing. The flight manual requires two pilots or, if operating with a single pilot, 2,100m of landing surface. With a bit of guesswork about how much power I was going to use, and prompting from Lappos, I could adjust the manual engine to stay always below the good engine, but still contribute significant power. I came to the hover, lowered the aircraft for the wheels to just touch the runway, reached up and pulled back the manual speed select to nominal torque and landed, with the manual engine still behind the governed engine. This was one of the advantages of the collective lever friction device. So, despite the flight manual, after training and practice, a single pilot should be able to perform similarly.
We next did an autorotation at the minimum rate of descent speed of 75-80kt, which gave 2,100ft/min. There was plenty of bite in the flare at the bottom to slow us down, increase the rotor RPM and give us time to sort out the next few seconds, which in this case was a powered recovery to the hover. The engines came back in rapidly at this point. Engines-off landings would be best practised in simulators, which I attempted later. The rotor RPM limits are wide (91-115%) with transients to 68% during an actual engines-off landing and overspeed to 121%, although RPM during our autorotations was stable and stayed well within the normal limits without any adjustment from the pilot.
An engine fire brings on a continuous audio tone with warning lights, including one in the end of the T-handle. There are other audio warnings for such events as undercarriage up if the speed is below 60kt and if rotor rpm is too low. The emergency procedure is straightforward with no likelihood of shutting down the wrong engine. The T-handle with the light on is bought aft all the way. During its travel, it collects the speed select lever and fuel shut-off lever, closing both. This action arms the fire extinguisher system and selects the correct engine. All the pilot needs to do now is fire the bottle.
After shutdown, Lappos interrogated the digital readout which records many parameters such as engine cycles, time at contingency power, faults and their ramifications. We had experienced a minor N1 indication problem. The system explained that it was not serious.
The S-76C+ is an example of a manufacturer responding to customer demand and getting it right, offering a safe, fast, long range and reliable helicopter with a modern cockpit which is easy to manage even for the single pilot in instrument conditions. It also offers passenger comfort, acceptable noise and good efficiency. The built-in training mode allows a high standard of pilot performance.
Sikorsky has calculated total direct costs as $669/h, plus crew costs. These will vary with area and operator. A utility S-76C+ costs $6 million, an executive model $7.7 million.
Source: Flight International
